The British actor, best known for his role as hard-drinking detective Jimmy McNulty on "The Wire," had his fill of the darker side while filming "Appropriate Adult" (9 p.m. Saturday, Sundance Channel; 3 stars). In the movie, he plays real-life serial killer Fred West (no relation), who with his wife Rosemary killed at least 12 young women and girls in Gloucester, England.

The 1994 case became one of the biggest police investigations in British history and changed the life of Janet Leach, a social worker in training who sat in on police interrogations as an "appropriate adult," a role in the British legal system meant to safeguard the rights of suspects in police custody.

The film focuses Fred's psychological grip over Janet (played by Emily Watson) as opposed to the murders. It's still a grisly tale, feuled by West's mesmerizing performance.

West was able to listen to the recordings if the interrogations, and said his performance was the result of that research. "After you've listened to a guy for about 50 hours you tend to just know how he thinks and clicks and speaks and what makes him tick," he said.

What West noticed most about Fred was his ability to manipulate those around him--and not just Janet.

"In photographs of him with police, the police are often smiling," West said. "There was a disconnect there. He's talking about chopping up his daughter like chopping wood or putting the trash out."

West said he has happy filming took just four weeks, because he didn't want to spend any more time plumbing Fred's psyche. And as to whether he will do another "pretty dark and pretty intense" psycho role, he was quick to answer.

"I was offered another one actually," he said. "But I think I've done that. So no."

West, who by now has donned the classic 1950s suits of Hector Madden to film Season 2 of BBC America’s “The Hour,” talked more about that series, finding Fred’s dark depravity in “Appropriate Adult” and about how Eminem’s love for “The Wire” resulted in West doing a voice in the “Dr. West” skit on “Relapse.”

SPOILER ALERT: If you don't know the details of the Fred West case, some answers could be considered spoilers.

What’s it like to embody Fred West, such a disturbing person, for weeks on end?Fortunately it was a fairly short filming period in that I did all my scenes back-to-back. So I was only at it for about three-and-a-half, four weeks. And it was away from home in Manchester and it was something that was [bleeping] intense for three weeks, but then I was able to leave it behind quite quickly and then I went straight into rehearsals for a play I was doing. So I got away from it quite easily, but at the time it was pretty dark and pretty intense.

It shows onscreen. The whole movie is pretty chilling. Did you do a lot of research?I read all the books about him, which in itself is initially—I mean I've never read books like that before. I know lots of people do and it’s surprising how easy you get into books about people like that, but I was able to listen to some of the hundred-odd hours of the real police interviews with Fred West and that’s an extraordinary resource really … That was the main thrust of my research and the performance was based entirely on those tapes really.

Those tapes must have been shocking to hear.You hear it enough it almost becomes normal—well normal isn’t the right work—but maybe I became sort of able to understand him better. It became one’s initial revulsion fading away, I suppose. I suppose inevitably it has to, although I was quite vigilant he wasn’t going to take me over and mess me up because people like that, even thought they’re dead, their malignancy does go on and on. And I know that from people who have written about him. And even though I had a very indirect experience of him it does mess you up and so I was pretty vigilant about that. I was pretty vigilant that he wasn’t going to mess me up.

Did you find him extremely manipulative of Janet on those tapes?That is really what the film is about; it’s not about him and his crimes. He’s a sort of background character, but what you see is the manipulation—the manipulative, malignant, manipulative nature of him. I did a few visits to prisons—mostly for “The Wire” in fact because it’s watched a lot in prisons—and I met quite a few criminals who committed sex crimes and horrible crimes. The thing that strikes you first is how insidiously manipulative they are and how good they are at making something sound innocent when it has an underlying motive.

There are times in the film when maybe think he’s not that bright and he has sort of been manipulated himself, but then you realize he just did that to Janet.I think there is a grain of truth in that, in that any time someone shows remorse, which he did, normal human beings or compassionate human beings feel compassion for them. That is what is sort of complex and sort of dramatic about the subject matter is that one does feel and she did feel—the appropriate adult—did feel compassion for this monster. You sort of have to pull yourself up short when you do find yourself feeling sorry for the bastard.

Did you meet the real Janet? And how is she dealing with this now, years later? Does it still trouble her?Absolutely. I mean he ruined her life. He ruined everyone’s lives, anyone who came near him. She felt that the film might sort of give her some closure on it and release her from it. I remember being skeptical at the time when she said it, but I think actually it has. I think actually her motives for what she did … have been vindicated really in that even if they failed she was trying to do good, she was trying to give some comfort to his victims and their families in getting him to confess.

Do you think that he took a lot of secrets to the grave?I think he certainly did, yeah. He certainly did. Martin Amis, who cousin was killed by him, as a “colossus of mendacity.” When you are that adept a liar, or that experienced a liar, then you probably can’t distinguish truth from reality yourself. I think he probably couldn’t. He was so dishonest his default position was to tell a lie and to be dishonest. I think he probably found it difficult to separate reality from a lie anyway, but he certainly knew an awful lot, which he wasn’t prepared to tell anyone. … I think there were a lot more people involved. His brother hanged himself. Another guy hanged himself. I think there was a pedophile ring. It was a circle of people involved in these. He wasn’t alone. I don’t think he was alone, but the secret has gone to the grave and we’ll never know.

Right, his wife will likely never talk. She seemed even harder than he did.Absolutely. She still claims her complete innocence. And it got so bad with him to the extent that he actually said, “I'll give you one body a year, so that I get one day out of prison,” to identify or to go and pinpoint the spot where he buried them. I mean that is the depth of his depravity, you know?

It’s curious that he killed himself.I think he was getting a hard time in prison and realized that was going to be for the rest of his life. I think also weirdly and paradoxically and strangely, the love or the obsession with his wife [led to it]. When she rejected him and refused to answer his letters and look at him in court, I think it did destroy him. I think it did. No matter how sick the relationship was it was something that meant everything to him.

So Fred couldn’t be any different than Hector Madden in “The Hour,” your other big role that played in the U.S. this year.[Laughs.] Yeah, no, well I hope not. [Laughs.] I hope they’re different. I've got to play a hero and a nice guy. I’m starting Hector again next week. We’re doing the second season next week, so I hope that shows me in a different light.

Is playing Hector easier on the old psyche?Yeah, it’s great fun. I just get wear nice suits and wander around with beautiful women like Romola [Garai] and Oona Chaplin, so. [Laughs.]

Well that was a great role, too, and a great miniseries. It aired on BBC America here.I'm glad you saw that. That’s great. Well we’re starting it again, so hopefully it will get better this time.

Good. You’ve also done a bunch of other work recently. “Arthur Christmas” is in theaters here; you did voice work for that. Do you always look for varied roles? How do you go about choosing your projects?Well not consciously, no. It’s just—maybe I do. I got offered a lot of cops after “The Wire” and I thought, “I don’t want to ever do that again.” The only criterion really is, “Does the part challenge and excite me?” and actually if I've done it before then it probably doesn’t excite me. So it’s not a conscious thing, but it’s nice to have a variety.

I read somewhere that you said you don’t really like talking about “The Wire” that much anymore.It’s not that I don’t like talking about it, but there’s nothing left about it I can think to say. I do like talking about it because I'm very fond of it and because it is the thing I'm most associated with and something I'm very proud of, but I've just been talking to a guy for a half an hour about it actually because it’s the 10th anniversary next year of the start of it and I suppose after 10 years one should shut up about it. [Laughs.]

I talked to Michael K. Williams recently and he said it’s sort of the gift that keeps on giving because people are still discovering it on DVD.That’s totally true. I've just come back from India and it’s very big in Delhi I'm delighted to tell you.

I find less and less people are going, “My God I've just watched ‘The Wire’ and it’s great.” It tends to be people going, “My God, I've just watched ‘The Wire’ for the fourth time and it’s still great!” I think in that way it’s a classic in that it does seem to be outlasting its ordinary shelf life.

Did I read that you’re doing a play with one of your “Wire” cohorts?I just did “Othello” with Clark Peters, who plays Lester Freeman in “The Wire,” and he played Othello. I played Iago, so I played another bad guy. It was summer of villainy in fact. We had an amazing time and hopefully we’re going to bring it to Broadway or London. It was a good show and we really loved doing it.

Do you keep in touch with cast members from “The Wire”?Yeah, I do. They’re all godparents to my children. I do. I'm very fond of them and they’ll be friends for life I hope.

I wanted to ask you about working with Eminem on “Dr. West.” How did that happen?He’s one of those fans who have watched “The Wire” four times. I said, “Good, but you ought to get out more.” [Laughs.]

He rang me up, amazingly, and he said his doctor when he was in rehab was an English guy and he wanted to write a sketch about him for his album and would I like to play the doctor. He said, “You start off as a doctor and then your voice morphs into Slim Shady.” I thought, “[Bleep] me; I’d love to do that!”

I love Eminem. I think he is brilliant and I had a great time doing it. He was in Miami and I was in Chiswick. [Laughs.] I thought I’d fly out there to be with him, but unfortunately technology allows you not to do that now. But I had a great time and then he came to London and I hooked up with him and we went out one evening and had a good time chatting. So I'm a big fan of his.

That’s so cool. You have done a lot of accents. What is the sort of easiest one and the most difficult?It seems to work one of two ways, which is, one, they give you the whole character and you just don’t think about the accent because you’re just playing the character. That’s what happened with Fred West, really, I went to Gloucester and listened to the accent a lot. The accent came very easily. But McNulty’s accent was the hardest. It was a constant battle that one, because I suppose it went on for so long and because I was using my own accent when I wasn’t onset. I found that very difficult and I don’t know why really. I was surrounded by people who had that accent and I was the only one who didn’t. I don’t know what it was, but I found it very difficult to sustain it.

But you did it anyway.I did it anyway.

You fooled everyone.I did it anyway, or the guy that got in to redub me did a great job. [Laughs.]

Well good job. And it was a great performance in “Appropriate Adult.” I won’t say that it made me feel good or anything, but it was a great performance. You scared me. How about that?[Laughs.] I’ll take that. That’s great.